In their first face-to-face negotiations in nearly seven years of war, Nicaraguan government officials and U.S.-backed Contras met for more than two hours Thursday to lay out sharply different truce proposals.

Msgr. Bosco Vivas, a Nicaraguan Roman Catholic bishop and the primary mediator at the talks, said the two sides were "sincere" and showed "mutual respect," but he expressed little hope for a quick cease-fire. Costa Rican officials were similarly pessimistic.

Vivas said both sides read their truce proposals aloud, then exchanged initial reactions. They agreed to meet again today, he said.

Proposal Outlined

Before the meeting began at a Roman Catholic seminary in a San Jose suburb, the negotiators representing Nicaragua's ruling Sandinista government unveiled their latest offer for a cease-fire by March 15 to be monitored by both an international commission and a "mixed military commission" of Nicaraguan soldiers and Contras.

The Sandinistas said their proposal offers the rebels guarantees of physical and political security but it maintains the government's longstanding position that the Contras must disarm before they will be allowed to discuss political reforms with the government as part of a civic opposition.

The Contras' proposal insists on sweeping constitutional reforms to limit the power of the leftist Sandinistas as a condition for a cease-fire.

Costa Rican officials said they do not expect major progress toward a cease-fire in the Nicaraguan guerrilla war before Congress votes next week on the Reagan Administration's proposal for $36.25 million in non-lethal and military aid for the Contras.

"The Contras will not want to negotiate in depth until the vote, and the Sandinista government will not want to make major promises before the vote," said a Costa Rican official who spoke on condition of anonymity. "The key to the game is not here, but in Washington."

In separate press conferences, the Contras charged that the Sandinista proposal was aimed at defeating the aid, and the Sandinistas contended that the rebel position was based on Reagan Administration policy rather than the Central American peace plan signed last August.

The talks are the result of that peace accord, conceived by Costa Rican President Oscar Arias Sanchez and signed by all five Central American presidents. The plan commits the governments to cease-fires, amnesty, political dialogue and democratic reforms to end the region's wars.

Throughout Nicaragua's war, the Sandinistas had rejected talks with the Contras and insisted on a direct dialogue with the United States, which helped found the insurgency and continues to fund it. Nicaraguan President Daniel Ortega agreed to send a negotiating team to the direct talks during a meeting of the Central American presidents held here earlier this month to judge the progress of the peace plan.

Talks Unsuccessful

The two sides held indirect talks twice in the Dominican Republic late last year, mediated by Nicaragua's Roman Catholic Cardinal Miguel Obando y Bravo. Those meetings were unsuccessful.

Thursday morning, before the talks here began, the Sandinista and Contra negotiating teams each met separately with the mediators, Bishops Vivas and Carlos Santi, at the rambling Central Seminary in the lower-middle-class Paso Ancho suburb. A knot of anti-Sandinista protesters gathered outside the 1950s-era structure, which was guarded by Costa Rican security personnel.

The Sandinista commission is headed by Deputy Foreign Minister Victor Hugo Tinoco and the army intelligence chief, Maj. Ricardo Wheelock; the Contra team is headed by banker Jaime Morales and guerrilla commander Walther Caldron.

Each side has two foreign observers. The Sandinistas' are American attorney Paul Reichler and West German Social Democrat Hans-Jurgen Wischnewski; the Contra advisers are two former foreign ministers, Gonzalo J. Facio of Costa Rica and Adm. Ramon Emilio Jimenez of the Dominican Republic.

More Talks Today

Thursday's talks began at about 4 p.m. local time in the seminary library, recessed briefly for coffee, and resumed until about 6:30. A second three-hour round is scheduled for today, and Vivas said still further talks might be scheduled.

Asked if it was possible to reach agreement in only two days, Vivas said, "We are hopeful, but we are not dreamers. . . I sincerely believe there will not be time to reach an agreement."

Contra negotiator Morales called the talks "a good start. There was no agreement or disagreement." A Nicaraguan Foreign Ministry official, Oscar Tellez, said the talks are "going well."

Both sides presented proposals that were fundamentally the same as those they offered during the indirect talks in Santo Domingo.

The Sandinistas argue that the peace plan requires them only to negotiate political reforms with an unarmed opposition. Under the Sandinista proposal, the army would stop offensive operations two weeks before a cease-fire began so the rebels could gather in designated cease-fire zones. The rebels would keep their guns during the monthlong cease-fire, which would begin by March 15.

Subject to Attack

Government officials and Sandinista "law enforcement" officials would be allowed to operate in the cease-fire zones. Armed rebels outside the zones would be subject to army attack.

An international commission and a mixed military commission would oversee the cease-fire in each zone and address violations.

Managua's proposal incorporates a recent offer of amnesty for political prisoners and an offer to establish an international commission, including members of the U.S. Republican and Democratic parties, to monitor the political rights of disarmed Contras. The government has promised amnesty for 3,300 prisoners once the war is over, or earlier if a country outside Central America will accept them.

Zones Are Negotiable

The Sandinistas submitted their earlier proposal for three cease-fire zones totaling more than 4,000 square miles but said the zones are negotiable. They also said the two sides could negotiate an extension of the cease-fire.

Sandinista observer Reichler said Managua's proposal "guarantees the Contras' physical and material security." Tinoco added that the proposal is meant to show the Contras that a cease-fire "is not a trap. The Administration has said ours is a proposal for the Contras' surrender, but that is not true. It is a proposal to open political space."

The Contras' proposal calls for new U.S. military aid to be placed in a 30-day escrow account while the government holds three-sided talks on constitutional reforms with the Contras and the internal political opposition.

Rebels Want Reforms

By the end of the 30-day period, a cease-fire would begin simultaneously with implementation of the reforms. The rebels are insisting on a list of 17 reforms sought by civic opposition parties that would separate the Sandinista National Liberation Front from the army and government institutions, weaken presidential power, abolish a second presidential term and guarantee private property.